1. Field of the Related Art
The present disclosure relates to a method and system for color management in image/text printing or display systems, and more particularly, to a method and system for achieving out-of-gamut spot color reproduction through use of a gamut mapping editor.
2. Background of the Related Art
To meet customer demand, the commercial printing industry requires the capability of producing spot colors accurately and consistently. Spot colors can be defined as a fixed set of colors which may be Pantone® colors, customer logo colors, colors in a customer's proprietary marked patterns, or customer defined colors in the form of an index color table. Spot colors are often used, or can be used, for large background areas, which may be the most color critical portion of a particular page. Consistent color in these areas may determine the difference between success and failure in meeting customer requirements.
Since imaging can occur over a variety of different printing systems and practiced by a variety of different clients and customers, the colors may not always be consistent or accurate. Existing spot color editors utilize a manual approach to the adjustment of CMYK (cyan, magenta, yellow, black) recipes of spot colors prior to raster image processing (Wing). For example, a document creator may select a Pantone® color for application in specific areas through a user interface on a printing device or computer monitor, such as that available on a Xerox® DocuSP® Controller. The Pantone® provided CMYK recipe for the selected printer is obtained from a look-up table. Prior to RIPing the document in the printer, the operator has the option of entering a spot color editor function and specifying an alternative CMYK recipe to achieve the desired color. The document is then RIPed and printed using the spot color editor recipes where specified, and Pantone® recipes otherwise.
Thus, spot color reproduction is a desirable property in the printing industry. There are different methods to reproduce accurate spot colors on a printer system, such as ICC (international color consortium) profile, iterating printer model, or directly iterating on a printer. However, regardless of what methods are used, only in-gamut spot colors may be reproduced accurately. For out-of-gamut spot colors, some gamut mapping methods have to be applied to map these colors onto a gamut surface. In such a case, a noticeable color difference between the target spot color and the final reproduction cannot be avoided.
Currently, most DFE (digital front end) servers do not inform customers whether the target spot colors are in-gamut or out-of-gamut. Thus, when customers observe color differences between reproduced and target spot colors, they are unaware of whether the difference is mainly caused by inaccurate spot color rendering or by the gamut mapping for this unachievable out-of-gamut target color. For the latter case, it may take customers plenty of time and resources to adjust CMYK recipes for the better matching of these unachievable out-of-gamut spot colors.
There are several gamut mapping methods that exist. For example, an out-of-gamut spot color can be mapped to various colors on the gamut surface based on different gamut mapping methods. Some methods reserve both hue and lightness, but sacrifice chroma, some methods reserve hue, but compromise lightness and chroma, and some gamut mapping methods attempt to locate the nearest point on the gamut surface in the CIELAB space or to minimize a color difference between the mapped and target colors under a specific color difference formula. One specific gamut mapping method may be suitable for some out-of-gamut spot colors, but not for others. For example, the DeltaE2000 gamut mapping method minimizes the color difference using a DeltaE2000 color difference formula, but it does not reserve hue values. This method can be used for the out-of-gamut spot colors near the gamut boundary. However, if it is applied to a spot color far from the gamut boundary, it may map the spot color to a color that has a quite different hue from the target color, and causes large visual color difference and unacceptable results. Another issue for using a fixed gamut mapping method for spot color reproduction is that several spot colors would be mapped to the same point on the gamut surface. That would cause these spot colors to be undistinguishable on prints.
Currently, most DFE servers provide one fixed gamut mapping method for all spot colors. Although this method is carefully selected to be suitable for most spot colors, it cannot satisfy all spot colors and the customer's requirements for highly accurate spot color reproduction. One method to apply various gamut mapping methods for spot colors is to use different ICC profiles for different spot colors. But this method still presents undesirable results. For example, if two spot colors are in the same page, only one ICC profile can be applied to these two spot colors.